Variants of the α1 subunit of human AMPK

ABSTRACT

Variants of the α subunit of AMP-activated kinase (AMPK), nucleic acids encoding such variants, and methods for their use are provided. The AMPK variants include splice variants that have an additional exon in the AMPK mRNA, and thus have an additional 15 amino acid residues in the encoded polypeptide. The AMPK variants can, for example, have at least 75% identity to the amino acid sequence of SEQ ID NO:4, and can contain an amino acid sequence having at least 75% identity to the amino acid sequence of SEQ ID NO:2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application under 35 U.S.C. § 371 and claims benefit under 35 U.S.C. § 119(a) of International Application No. PCT/IB03/00807 having an International Filing Date of Jan. 31, 2003, which claims the benefit of priority of U.S. Provisional Application Ser. No. 60/353,406 having a filing date of Feb. 1, 2002.

TECHNICAL FIELD

The invention relates to protein kinases, and more particularly to variants of the alpha subunit of the human AMP-activated protein kinase.

BACKGROUND

The AMP-activated protein kinase (AMPK) acts as an intracellular metabolic sensor in a variety of cells, where it monitors and responds to variations in the AMP:ATP ratio (Hardie et al., Annu. Rev. Biochem. 67:821–855, 1998). Upon activation of AMPK, the enzyme phosphorylates a number of protein substrates to decrease further ATP usage by the cell. AMPK is a heterotrimeric complex consisting of a catalytic subunit (α) and two associated subunits (β and γ). Both the β and γ subunits are required for optimal activity of the α catalytic subunit. The AMPK complex is evolutionarily conserved and also can be found in yeast and plants. Mammalian AMPK is composed of different isoforms of subunits: α1, α2, β1, β2, γ1, γ2, and γ3 (Hardie and Hawley, BioEssays 23:1112–1119, 2001). Different combinations of isoform subunits are activated differently in vivo, and most likely also differ in substrate utilization. AMPK activity is found in several tissues, including liver, kidney, muscle, lung, and brain (Cheung et al., Biochem. J. 346:659–669, 2000).

AMPK is recognized as a major regulator of lipid biosynthetic pathways due to its role in the phosphorylation and inactivation of key enzymes such as acetyl-CoA carboxylase (Hardie and Carling, Eur. J. Biochem. 246:259–273, 1997). More recent work has suggested that AMPK has a wider role in metabolic regulation (Winder and Hardie, Am. J. Physiol. 277:E1–10, 1999); this includes fatty acid oxidation, muscle glucose uptake, expression of cAMP-stimulated gluconeogenic genes such as PEPCK and G6Pase, and expression of glucose-stimulated genes associated with hepatic lipogenesis, including fatty acid synthase, Spot-14, and L-type pyruvate kinase. Chronic activation of AMPK also can induce the expression of muscle hexokinase and glucose transporters (Glut4), mimicking the effects of extensive exercise training (Holmes et al. J. Appl. Phiysiol. 87:1990–1995, 1999). The activation of AMPK thus might be a good approach to treat type 2 diabetes; this hypothesis is supported by the finding that AMPK is the target for metformin, a drug widely used to treat type 2 diabetes (Zhou et al. J. Clin. Invest. 108:1167–1174, 2001).

SUMMARY

The invention is based on the identification of variants of the α1 subunit of human AMPK, including splice variants that result in inclusion of an additional exon in the AMPK mRNA and an additional 15 amino acid residues in the encoded polypeptide. Such a polypeptide can regulate the activity of the AMPK trimer or alter the substrate specificity of the kinase.

In one aspect, the invention features a purified polypeptide that includes a splice variant of the α1 subunit of human AMPK. The purified polypeptide can contain the amino acid sequence of SEQ ID NO:2, or can contain an amino acid sequence having at least 75% identity to the amino acid sequence of SEQ ID NO:2. The purified polypeptide can have the amino acid sequence of SEQ ID NO:4.

The invention also features an isolated nucleic acid encoding a polypeptide, wherein the polypeptide is a splice variant of the α1 subunit of human AMPK. The isolated nucleic acid can contain a nucleotide sequence at least 75% identical to the nucleotide sequence of SEQ ID NO:1. The isolated nucleic acid sequence can have the nucleotide sequence of SEQ ID NO:3. The invention also features an expression construct containing such nucleic acids.

In anther aspect, the invention features a method for identifying an agent capable of modulating the activity of a polypeptide, wherein the polypeptide is a splice variant of the α1 subunit of human AMPK. The method can include contacting a candidate compound with the polypeptide or with a plurality of cells expressing the polypeptide, measuring the effect of the candidate compound on the activity of the polypeptide, and identifying the candidate compound as an agent capable of modulating the activity of the polypeptide if the activity is increased or decreased in the presence of the compound.

The invention also features a method for identifying an agent capable of modulating the activation of a polypeptide, wherein the polypeptide is a splice variant of the α1 subunit of human AMPK. The method can include contacting a candidate compound with the polypeptide or with a plurality of cells expressing the polypeptide, measuring the effect of the candidate compound on the activation of the polypeptide, and identifying the candidate compound as an agent capable of modulating the activation of the polypeptide if the activation is increased or decreased in the presence of the candidate compound.

The invention features a method for identifying an agent capable of modulating the amount of a polypeptide produced by a cell, where the polypeptide is a splice variant of the α1 subunit of human AMPK. The method can involve contacting a candidate compound with a plurality of cells expressing the polypeptide, measuring the effect of the candidate compound on the amount of the polypeptide produced by the cells, and identifying the candidate compound as an agent capable of modulating the amount of the polypeptide produced by the cell if the amount is increased or decreased in the presence of the candidate compound.

In yet another aspect, the invention features an isolated antibody having specific binding affinity for a polypeptide, where the polypeptide is a splice variant of the α1 subunit of human AMPK.

The invention also features an isolated nucleic acid probe that specifically hybridizes to a nucleic acid encoding a splice variant of the α1 subunit of human AMPK or the complement of the nucleic acid. The nucleic acid probe can specifically hybridize to the nucleic acid sequence of SEQ ID NO:1 or a sequence complementary thereto.

The invention also features a method for specifically detecting the presence of a polypeptide in a sample, where the polypeptide is a splice variant of the α1 subunit of human AMPK. The method involves contacting the sample with an antibody described above.

The invention also features a method for specifically detecting the presence of an AMPK heterotrimer complex in a biological sample containing a polypeptide, where the polypeptide is a splice variant of the α1 subunit of human AMPK. The method can include contacting the biological sample with an antibody described above.

The invention also features a method for specifically detecting the presence of a nucleic acid encoding a splice variant of the α1 subunit of human AMPK. The method can involve contacting a biological sample with a nucleic acid probe described above.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used to practice the invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

The details of one or more embodiments of the invention are set forth in the accompanying description below. Other features, objects, and advantages of the invention will be apparent from the description and, from the claims.

DETAILED DESCRIPTION

In general, the invention provides polypeptides that are splice variants of the α1 subunit of AMPK and nucleic acids encoding such polypeptides. As described herein, a polypeptide splice variant of the α1 subunit of AMPK contains an additional 15 amino acid residues and can result in an AMPK trimer having a different structural surface than an AMPK trimer containing an α1 subunit that lacks these extra amino acid residues. This may have a direct effect on substrate binding to AMPK. For example, a protein that normally functions as a substrate for AMPK may not retain the ability to function as a substrate, possibly because new interactions prevent the substrate from fitting properly into the substrate pocket of the kinase. Since AMPK is known to have different substrates that are involved in different signaling and metabolic pathways, the presence of a variant of the α1 subunit could alter the activity of the kinase in different signaling and metabolic pathways.

A structural model of the core domain of another kinase (Engh and Bossemeyer, Advan. Enzyme Regul. 41:124, 2001) indicates that the substrate pocket of a kinase consists of two parts, a small substrate pocket and a large substrate pocket. The small substrate pocket typically is where the substrate amino acid residue to be phosphorylated interacts with the kinase. The large substrate pocket is defined as the bigger part of the cleft between the two subdomains of a kinase core domain, where the rest of the substrate protein interacts with the kinase during a phosphorylation event or where the substrate protein would be in close proximity to the kinase. Without being bound by a particular mechanism, the polypeptide sequence encoded by the additional exon identified herein can lie on one side of the larger substrate pocket of the kinase cleft. More specifically, this polypeptide sequence can be found between alpha helix D and alpha helix E (as defined in Engh and Bossemeyer, supra). This could either extend the two existing helixes or protrude in an enlarged loop linking the two helixes. The function of polypeptide sequence encoded by the additional exon could, in an AMPK trimer complex: (i) alter the substrate specificity of the kinase, and/or (ii) have a regulatory effect on the activity of the kinase.

The presence of a variant of the α1 subunit in an AMPK kinase trimer also could potentiate the use of a new substrate. The presence of a variant of the α1 subunit could provide the structural requirements needed for a protein that is otherwise not a substrate for AMPK to be able to bind to the kinase and thus function as a substrate.

Variants of the α1 subunit of human AMPK could have a role in regulation of the activity of the AMPK trimer. For example, an α1 subunit variant could affect the ability of AMP to activate the kinase, by directly or indirectly changing the sensitivity for the kinase to AMP. AMPK is activated by phosphorylation of an upstream AMPK-kinase (AMPKK) and the presence of a variant of the α1 subunit could affect, by steric constraints, the ability of AMPKK to phosphorylate AMPK. Furthermore, the presence of a α1 subunit variant could change, also by steric considerations, the regulation of AMPK activation by dephosphorylation.

Polypeptides

The present invention provides polypeptides that are variants of the α1 subunit of human AMPK. A “polypeptide” refers to a chain of amino acid residues, regardless of post-translational modification (e.g., phosphorylation or glycosylation). As used herein, “variants of the α1 subunit of human AMPK” have an amino acid sequence that differs from the amino acid sequence of the wild type human AMPK α1 subunit (see GenBank Accession No. NM_(—)006251). Variants can result from, for example, a substitution, deletion, or insertion within the wild type amino acid sequence.

A variant can be, for example, a naturally occurring splice variant of the human AMPK α1 subunit. In one embodiment, a splice variant includes an amino acid sequence encoded by additional exon sequences of the α1 subunit of human AMPK. The additional exon sequence can be located, for example, between exon 3 and exon 4 of the gene encoding the α1 subunit of human AMPK. The additional exon sequence can be the sequence set forth in SEQ ID NO:1.

The present invention further provides variants of the α1 subunit of human AMPK containing the amino acid sequence shown in SEQ ID NO:2 or an amino acid sequence that is at least 75% identical (e.g., at least 80% identical, at least 90% identical, or at least 95% identical), to a polypeptide containing the amino acid sequence of SEQ ID NO:2. The variant can have, for example, the amino acid sequence shown in SEQ ID NO:4.

To determine the percent identity of two amino acid sequences or of two nucleic acids, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in the sequence of a first amino acid or nucleic acid sequence for optimal alignment with a second amino or nucleic acid sequence). The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, then the molecules are identical at that position. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e., percent identity=number of identical positions/total number of positions (e.g., overlapping positions)×100). In one embodiment, the two sequences are the same length.

To determine percent sequence identity, a target nucleic acid or amino acid sequence is compared to the identified nucleic acid or amino acid sequence using the BLAST 2 Sequences (B12seq) program from the stand-alone version of BLASTZ containing BLASTN version 2.0.14 and BLASTP version 2.0.14. This stand-alone version of BLASTZ can be obtained from Fish & Richardson's web site (world wide web at fr.com/blast) or the U.S. government's National Center for Biotechnology Information web site (world wide web at ncbi.nlm.nih.gov). Instructions explaining how to use the B12seq program can be found in the readme file accompanying BLASTZ.

B12seq performs a comparison between two sequences using either the BLASTN or BLASTP algorithm. BLASTN is used to compare nucleic acid sequences, while BLASTP is used to compare amino acid sequences. To compare two nucleic acid sequences, the options are set as follows: -i is set to a file containing the first nucleic acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second nucleic acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastn; -o is set to any desired file name (e.g., C:\output.txt); -q is set to −1; -r is set to 2; and all other options are left at their default setting. The following command will generate an output file containing a comparison between two sequences: C:\B12seq -i c:\seq1.txt -j c:\seq2.txt -p blastn -o c:\output.txt -q −1 -r 2. If the target sequence shares homology with any portion of the identified sequence, then the designated output file will present those regions of homology as aligned sequences. If the target sequence does not share homology with any portion of the identified sequence, then the designated output file will not present aligned sequences.

Once aligned, a length is determined by counting the number of consecutive nucleotides from the target sequence presented in alignment with sequence from the identified sequence starting with any matched position and ending with any other matched position. A matched position is any position where an identical nucleotide is presented in both the target and identified sequence. Gaps presented in the target sequence are not counted since gaps are not nucleotides. Likewise, gaps presented in the identified sequence are not counted since target sequence nucleotides are counted, not nucleotides from the identified sequence.

The percent identity over a particular length is determined by counting the number of matched positions over that length and dividing that number by the length followed by multiplying the resulting value by 100. For example, if (1) a 50 nucleotide target sequence is compared to the sequence set forth in SEQ ID NO:1, (2) the B12seq program presents 45 nucleotides from the target sequence aligned with a region of the sequence set forth in SEQ ID NO:1 where the first and last nucleotides of that 45 nucleotide region are matches, and (3) the number of matches over those 45 aligned nucleotides is 40, then the 50 nucleotide target sequence contains a length of 45 and a percent identity over that length of 89 (i.e., 40/45×100=89).

It will be appreciated that different regions within a single nucleic acid target sequence that aligns with an identified sequence can each have their own percent identity. It is noted that the percent identity value is rounded to the nearest tenth. For example, 78.11, 78.12, 78.13, and 78.14 are rounded down to 78.1, while 78.15, 78.16, 78.17, 78.18, and 78.19 are rounded up to 78.2. It also is noted that the length value will always be an integer.

Antibodies

The invention also provides antibodies that have specific binding affinity for a polypeptide variant of the α1 subunit of human AMPK. “Antibody” or “antibodies” includes intact molecules as well as fragments thereof that are capable of binding to an epitope of an AMPK α1 subunit. The term “epitope” refers to an antigenic determinant on an antigen to which an antibody binds. Epitopes usually consist of chemically active surface groupings of molecules such as amino acids or sugar side chains, and typically have specific three-dimensional structural characteristics, as well as specific charge characteristics. Epitopes generally have at least five contiguous amino acids. The terms “antibody” and “antibodies” include polyclonal antibodies, monoclonal antibodies, humanized or chimeric antibodies, single chain Fv antibody fragments, Fab fragments, and F(ab)₂ fragments. Polyclonal antibodies are heterogeneous populations of antibody molecules that are specific for a particular antigen, while monoclonal antibodies are homogeneous populations of antibodies to a particular epitope contained within an antigen. Monoclonal antibodies are particularly useful.

In general, an AMPK α1 polypeptide is produced, for example, by chemical synthesis or by purification of the native protein and then used to immunize animals. Various host animals including, for example, rabbits, chickens, mice, guinea pigs, and rats, can be immunized by injection of the protein of interest. Depending on the host species, adjuvants can be used to increase the immunological response and include Freund's adjuvant (complete and/or incomplete), mineral gels such as aluminum hydroxide, surface-active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanin, and dinitrophenol. Polyclonal antibodies are contained in the sera of the immunized animals. Monoclonal antibodies can be prepared using standard hybridoma technology. In particular, monoclonal antibodies can be obtained by any technique that provides for the production of antibody molecules by continuous cell lines in culture as described, for example, by Kohler et al. (1975) Nature 256:495–497, the human B-cell hybridoma technique of Kosbor et al. (1983) Immunology Today 4:72, and Cote et al. (1983) Proc. Natl. Acad. Sci. USA 80:2026–2030, and the EBV-hybridoma technique of Cole et al., Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc. pp. 77–96 (1983). Such antibodies can be of any immunoglobulin class including IgM, IgG, IgE, IgA, IgD, and any subclass thereof. The hybridoma producing the monoclonal antibodies of the invention can be cultivated in vitro or in vivo.

A chimeric antibody is a molecule in which different portions are derived from different animal species, such as those having a variable region derived from a mouse monoclonal antibody and a human immunoglobulin constant region. Chimeric antibodies can be produced through standard techniques.

Antibody fragments that have specific binding affinity for an AMPK α1 subunit polypeptide can be generated by known techniques. Such antibody fragments include, but are not limited to, F(ab′)₂ fragments that can be produced by pepsin digestion of an antibody molecule, and Fab fragments that can be generated by deducing the disulfide bridges of F(ab′)₂ fragments. Alternatively, Fab expression libraries can be constructed. See, for example, Huse et al. (1989) Science 246:1275–1281. Single chain Fv antibody fragments are formed by linking the heavy and light chain fragments of the Fv region via an amino acid bridge (e.g., 15 to 18 amino acids), resulting in a single chain polypeptide. Single chain Fv antibody fragments can be produced through standard techniques, such as those disclosed in U.S. Pat. No. 4,946,778.

Once produced, antibodies or fragments thereof can be tested for recognition of an AMPK α1 subunit by standard immunoassay methods including, for example, enzyme-linked immunosorbent assay (ELISA) or radioimmunoassay (RIA). See Short Protocols in Molecular Biology, eds. Ausubel et al., Green Publishing Associates and John Wiley & Sons (1992). Suitable antibodies typically have equal binding affinities for recombinant and native proteins.

Antibodies that have specific binding affinity for an AMPK α1 subunit variant can be used to detect variants of the α1 subunit of human AMPK in a biological sample. As used herein, a biological sample contains cells or cellular material, and can include, for example, urine, blood, cerebrospinal fluid, pleural fluid, sputum, peritoneal fluid, bladder washings, secretions, oral washings, tissue samples, touch preps, or fine-needle aspirates. Methods for detecting α1 subunit variants include contacting such a sample with an antibody of the invention. The variant of the α1 subunit can be, present in an AMPK heterotrimer complex.

Nucleic Acids

The present invention further provides isolated nucleic acid molecules encoding variants of the α1 subunit of human AMPK. Nucleic acids of the invention can contain nucleotide sequences that are at least 75% identical (e.g., at least 80% identical, at least 90% identical, or at least 95% identical), to the nucleotide sequence of SEQ ID NO:1. The nucleotide sequence of an isolated nucleic acid sequence may be, for example, the nucleotide sequence shown in SEQ ID NO:3.

The invention also provides vectors containing nucleic acid sequences encoding variants of the α1 subunit of human AMPK. As used herein, a “vector” is a replicon, such as a plasmid, phage, or cosmid, into which another DNA segment can be inserted so as to bring about the replication of the inserted segment. The vectors of the invention can be expression vectors. An “expression vector” is a vector that includes one or more expression control sequences, and an “expression control sequence” is a DNA sequence that controls and regulates the transcription and/or translation of another DNA sequence.

An expression vector containing a nucleic acid that encodes a variant of the human AMPK α1 subunit can be introduced into host cells by any of a number of techniques including, for example, calcium phosphate transformation, DEAE-dextran transformation, cationic lipid mediated lipofection, electroporation, or infection.

The α1 subunit can be expressed in a variety of hosts such as bacteria, plant cells, insect cells, fungal cells, and human and animal cells. Eukaryotic recombinant host cells are particularly useful. Nonlimiting examples include yeast, mammalian cells including cell lines of human, bovine, porcine, monkey, and rodent origin, and insect cells including Drosophila and silkworm derived cell lines. Examples of cell lines derived from mammalian species that are commercially available include: L cells L-M(TK-) (ATCC CCL 1.3), L cells L-M (ATCC CCL 1.2), HEK 293 (ATCC CRL 1573), Raji (ATCC CCL 86), CV-1 (ATCC CCL 70), COS-1 (ATCC CRL 1650), COS-7 (ATCC CRL 1651), CHO-K1 (ATCC CCL 61), 3T3 (ATCC CCL 92), NIH/3T3 (ATCC CRL 1658), HeLa (ATCC CCL 2), C127I (ATCC CRL 1616), BS-C-1 (ATCC CCL 26) and MRC-5 (ATCC CCL 171).

The invention further provides nucleic acid probes that can specifically hybridize to a nucleic acid encoding a variant of the α1 subunit of human AMPK or to the complement of the nucleic acid. Nucleic acid probes that specifically hybridize to the nucleotide sequences of SEQ ID NO:1 and SEQ ID NO:3, or sequences complementary thereto are particularly useful. Such nucleic acid probes are useful, for example, for amplifying and/or detecting a nucleic acid encoding a polypeptide of the invention.

“Specific hybridization” of a nucleic acid probe refers to a nucleic acid that hybridizes only to nucleic acids encoding variants of the α1 subunit of human AMPK, or the complements thereof, without hybridizing to nucleic acids encoding related polypeptides. Such hybridization typically is carried out under stringent hybridization conditions. The term “stringent” when used in conjunction with hybridization conditions is as defined in the art, i.e., 15–20° C. under the melting point Tm. Preferably the conditions are “highly stringent,” i.e., 5–10° C. under the melting point Tm. High stringency conditions can include the use of low ionic strength buffer and a high temperature for washing, for example, 0.015 M NaCl/0.0015 M sodium citrate (0.1×SSC), 0.1% sodium dodecyl sulfate (SDS) at 65° C. Alternatively, denaturing agents such as formamide can be employed during hybridization, e.g., 50% formamide with 0.1% bovine serum albumin/0.1% Ficoll/0.1% polyvinylpyrrolidone/50 mM sodium phosphate buffer at pH 6.5 with 750 mM NaCl, 75 mM sodium citrate at 42° C. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Molecular Cloning: A Laboratory Manual, 3rd ed., Sambrook et al. eds., Cold Spring Harbor Laboratory Press, 2001; DNA Cloning: A practical Approach, Glover & Hames eds., Oxford University Press, 1996; and Nucleic Acid Hybridization: Essential techniques, Ross ed. Wiley, 1998.

Methods

The invention provides methods for identifying a therapeutic agent capable of modulating the activity of a polypeptide, where the polypeptide is a variant of the α1 subunit of human AMPK. Such methods can include: (i) contacting a candidate compound with such a polypeptide or a plurality of cells expressing such a polypeptide; and (ii) measuring the effect of the candidate compound on the activity of the polypeptide. Modulating the activity of a variant of the α1 subunit of human AMPK can be achieved by, for example, modulating the level of phosphorylation, substrate specificity, AMP activation, substrate affinity, or resistance to protein phosphorylases. Modulating the activity of a variant of the α1 subunit of human AMPK can occur by either stimulation or inhibition. Agents that can stimulate the activity of a variant of the α1 subunit of human AMPK are particularly useful.

Methods of the invention also can be used to identify a therapeutic agent capable of modulating the activation of a variant of the α1 subunit of human AMPK according to the invention. AMPK can be allosterically activated by AMP, or activated by phosphorylation (e.g., in the activation loop of the α1 subunit). Thus, modulation of the activation of a variant of the α1 subunit of human AMPK can be achieved through, for example, modulating the level of phosphorylation, AMP activation, or resistance to protein phosphorylases. Methods for identifying such therapeutic agents can include:

-   (i) contacting a candidate compound with a variant of the α1 subunit     of human AMPK or a plurality of cells expressing a variant of the α1     subunit of human AMPK; and -   (ii) measuring the effect of the candidate compound on the     activation of the variant of the α1 subunit of human AMPK.     Modulation of the activation of a variant of the α1 subunit of human     AMPK can either increase or decrease the activation. Agents capable     of increasing the activation of a variant of the α1 subunit of human     AMPK are particularly useful.

Assays used to determine the effect of a compound to be tested on the activity or activation of a variant of the α1 subunit of human AMPK can be based on measurement of the in vitro phosphorylation by AMPK of synthetic peptide substrates as described, for example, by Davies et al. (Eur. J. Biochem. 186:123–128, 1989) and Michell et al. (J. Biol. Chem. 271:28445–28450, 1996).

Therapeutic agents that can modulate the amount of a variant of the α1 subunit of human AMPK produced by a cell also can be identified using methods of the invention. For example, a candidate compound can be contacted with a plurality of cells expressing a variant of the α1 subunit of human AMPK and the effect of the candidate compound on the amount of the variant of the α1 subunit of human AMPK that is produced by the plurality of cells can be measured. Modulation of the amount of a variant of the α1 subunit of human AMPK that is produced includes increasing or decreasing the amount of the polypeptide that is produced. Therapeutic agents capable of increasing the amount of a variant of the α1 subunit of human AMPK that is produced are particularly useful.

The amount of α1 subunit that is produced in cells can be altered, for example, by modulating transcription, splicing, or translation of nucleic acids encoding the variant of the α1 subunit of human AMPK. Assays used to determine the effect of a test compound on the amount of a variant of the α1 subunit of human AMPK can be based on:

-   (i) measurement of the amount of mRNA formed using, e.g., Northern     blot analysis or quantitative real time PCR, (ii) measurement of the     amount of protein formed using, e.g., Western blot analysis, or     immunochemical analysis such as ELISA, or (iii) measurement of     activity as described above, in cells expressing a variant of the α1     subunit of human AMPK.

Compounds identified by the methods described herein can be used to regulate metabolism. “Regulation of metabolism” as used herein refers to the ability of the therapeutic agent to mediate cell processes related to insulin resistance syndrome and other related disorders, such as non-insulin dependent diabetes mellitus, dyslipidemia, obesity and atherosclerosis.

The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.

EXAMPLES Example 1 Materials and Methods

Synthetic oligonucleotides were designed for use as primers to amplify and thus clone the coding region of the α1 catalytic subunit of human AMPK (GenBank accession number NM_(—)006251) by polymerase chain reaction (PCR). The sense primer (AM2sS) with the nucleotide sequence 5′-GAGCATGCAGATGGCGACAGCCGAGAA-3′ (SEQ ID NO:5; start codon underlined) contained a 5′ SphI restriction enzyme site and two extra bases on each end, while the anti-sense primer (AM2as) had the nucleotide sequence 5′-TTATTGTGCAAGAATTTTAATTAGAT-3′ (SEQ ID NO:6). First strand cDNA was synthesized from human skeletal muscle mRNA (M. quadriceps, iliopsoas, and pectoralis major from male and female; Clontech, Palo Alto, Calif.) using the SuperScript II system (Gibco/Invitrogen, Carlsbad, Calif.). PCR was performed using Advantage 2 polymerase (Clontech), a GenAMP PCR System 9600 instrument (Perkin Elmer, Wellesley, Mass.), and the following cycles: an initial denaturation at 95° C. for one minute; five cycles of 94° C. for 15 seconds, 60° C. for 15 seconds, and 72° C. for 2 minutes; 38 cycles of 94° C. for 15 seconds, 54° C. for 10 seconds, and 72° C. for 2 minutes; and one final segment of 72° C. for 3 minutes. A PCR product approximately 1.7 kb in length was purified from agarose gel. The ends of the PCR product were polished using a T4 DNA polymerase (Amersham Pharmacia Biotech, Piscataway, N.J.) for five minutes at room temperature and thereafter purified using the QIAquick PCR Purification Kit (Qiagen, Valencia, Calif.). The PCR product was digested with SphI (New England BioLabs, Beverly, Mass.) for 90 minutes, after which the cDNA fragment was once again purified. The digested PCR fragment was ligated into the SphI and SmaI sites of the bacterial expression vector pQE-32 (Qiagen) and subsequently transformed into electrocompetent DH10b bacteria (Gibco/Invitrogen). Overnmight cultures were started from individual colonies and mini plasmid preparation was performed using a QIAprep Spin miniprep Kit (Qiagen). DNA sequencing was performed on a MegaBACE 1000 DNA Analysis System (Amersham Biosciences) and DNA sequence analysis was performed using EditView (Applied Biosystems, Foster City, Calif.) and MacVector (Accelrys, Burlington, Mass.) software programs.

Example 2 Identification and Cloning of a Variant of the α1 Subunit of Human AMPK

One of the five clones (SEQ ID NO:3) that were isolated as described in Example 1 contained an internal extra nucleotide sequence of 45 bp, as compared to the published cDNA sequence for the human AMPK α1 subunit (GenBank #NM_(—)006251). These extra 45 bp did not alter the existing reading frame of the α1 subunit of AMPK.

The extra 45 bp correspond to a previously undescribed exon of the human AMPK α1 subunit, when compared to genome clone #AC008810. This new exon adheres to the AG-exon-GT consensus for exon/intron boundaries (International Human Genome Sequencing Consortium, Nature 409:860–921, 2001). The corresponding positions in the rat and mouse genes for the AMPK α1 subunit show significant sequence similarity to the human gene, but contain nucleotide substitutions, deletions, and insertions, as well as a lack of consensus motifs at exon/intron boundaries. These differences likely render the corresponding rat and mouse sequences incapable of forming any exon. The rat sequence was found in GenBank #AC094562, and the mouse sequence was obtained by PCR using mouse genomic DNA and AMPK α1 subunit specific primers.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. 

1. A purified polypeptide comprising a splice variant of the α1 subunit of human AMP-activated kinase (AMPK) having at least 95% identity to the amino acid sequence of SEQ ID NO:4, and comprising an amino acid sequence having at least 93% identity to the amino acid sequence of SEQ ID NO:2, and wherein said splice variant has kinase activity.
 2. The purified polypeptide of claim 1, wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:2.
 3. The purified polypeptide of claim 1, wherein said splice variant of the α1 subunit of human AMPK has the amino acid sequence of SEQ ID NO:4. 